A growing number of constitutional law scholars are arguing that Ted Cruz’s birth in Canada makes him ineligible to become U.S. president. Their argument could prove a thorn in the side of the senator, who is a zealous originalist on most constitutional questions—with what seems like a notable exception.

The issue has moved to the center of the presidential campaign, with Cruz’s rise in the polls and Donald Trump claiming that Cruz needs to prove he’s eligible to run by getting a declaratory judgment in federal court.

There is some ambiguity in the question of eligibility. The Constitution sets down three requirements to assume the nation's highest office: one must be at least 35 years old, have been a resident of the U.S. for at least 14 years (though whether those years must be consecutive or can be cumulative is a question up for debate) and must be a "natural-born citizen" of the United States. But the founders did not explicitly define "natural-born citizen," leaving room for doubt and debate.

While Cruz has told reporters his eligibility to become president is "settled law" because his mother was an American citizen when he was born and never renounced her American citizenship while she was a Canadian resident. Many constitutional theorists agree with Cruz that it's not really up for debate.

But it’s hardly unanimous. An increasing number of high-profile constitutional law professors, including one of Cruz's own professors from Harvard Law School, have in recent days argued publicly that Cruz's birth disqualifies him.

There are three leading theories of how to interpret the Constitution today. One is textualism: The Constitution means what its words say. The historical context of the words is important when a modern plain meaning is not self-evident. A second theory, adopted by many liberals, relies on a "living Constitution": the Constitution means what is most consistent with fundamental constitutional values as applied to present circumstances. The third theory, championed by many leading conservatives, is originalism: The Constitution means what ordinary people would have understood it to mean at the time it was ratified, in 1788.

According to Lee, two legal theories of citizenship were popular at the time the Constitution was ratified: jus soli (Latin for "law of the land), which held that a child's citizenship flowed from the actual, physical place of his birth, and jus sanguinis ("law of the blood"), which held that parents passed their citizenship to their children. However, Lee argues, at the time the Constitution was ratified, jus sanguinis applied only to patrilineal descent.

"However odious it seems today, a child born of a woman whose citizenship was different from her husband's—much rarer then than today—could not be a 'natural born Citizen' of the mother's country. That idea wasn't even considered until 1844 in Victorian England."

Mary Brigid McManamon, a constitutional law professor at Widener University, made a similar argument in The Washington Post Tuesday. "In this election cycle, numerous pundits have declared that Cruz is eligible to be president," she writes. "They rely on a supposed consensus among legal experts. This notion appears to emanate largely from a recent comment in the Harvard Law Review Forum by former Solicitors General Neal Katyal and Paul Clement. In trying to put the question of who is a natural-born citizen to rest, however, the authors misunderstand, misapply and ignore the relevant law."

The law Katyal and Clement are ignoring, McManamon argues, is 18th-century English common law, which the Supreme Court has said is a necessary lens for understanding the founders' understanding of the Constitution—a fact that Katyal, Clement and McManamon agree on. English common law was "unequivocal" on the subject, McManamon says: "Natural-born subjects had to be born in English territory." Katyal and Clement, rather than relying on common law, turn for their interpretation to a trio of 18th-century British statutes that were "a revolutionary departure" from the common law, McManamon argues.

Now a former teacher of Cruz's says he thinks the senator isn't eligible to run for president. Laurence Tribe, a professor of constitutional law at Harvard who taught both Cruz and President Barack Obama, wrote about the subject in an op-ed published Monday in The Boston Globe.

"People are entitled to their own opinions about what the definition ought to be," Tribe writes. "But the kind of judge Cruz says he admires and would appoint to the Supreme Court is an 'originalist,' one who claims to be bound by the narrowly historical meaning of the Constitution's terms at the time of their adoption. To his kind of judge, Cruz ironically wouldn't be eligible, because the legal principles that prevailed in the 1780s and '90s required that someone actually be born on U.S. soil to be a 'natural born' citizen. Even having two U.S. parents wouldn’t suffice. And having just an American mother, as Cruz did, would have been insufficient at a time that made patrilineal descent decisive."

Tribe later called Cruz a "fair-weather originalist" on CNN, saying the senator's philosophy is "antiquated...but it turns out Ted Cruz drops that when it doesn't serve his purpose."

Donald Trump, the front-runner for the Republican nomination, had been touting Tribe's scholarship on the question in the days leading up to the Harvard professor's op-ed. In response, Cruz called Tribe "a liberal left-wing judicial activist."